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please help!
- Thread starter ChrisieB
- Start date
Hello, ChrisieB!
You can't . . . The statement is not true.
\(\displaystyle \text{Let }x = 0:\;\;8\cos^40 \:=\:3 + 4\cos0 + 4\cos0 \quad\Rightarrow\quad 8 \:=\:11\;\;??\)
\(\displaystyle \text{How do i prove that: }\; 8\cos^4x \:=\:3 + 4\cos2x +4\cos x\)
You can't . . . The statement is not true.
\(\displaystyle \text{Let }x = 0:\;\;8\cos^40 \:=\:3 + 4\cos0 + 4\cos0 \quad\Rightarrow\quad 8 \:=\:11\;\;??\)
D
Deleted member 4993
Guest
If Cody's interpretation is correct - i.e. - you wanted to prove:
\(\displaystyle 8cos^4(x) \ = \ 3 \ + \ 4cos(2x) \ + \ cos(4x)\)
start working with
\(\displaystyle cos(4x) \ = \ cos(2*2x) \ = \ 2*cos^2(2x) \ - \ 1\)
and continue.....
\(\displaystyle 8cos^4(x) \ = \ 3 \ + \ 4cos(2x) \ + \ cos(4x)\)
start working with
\(\displaystyle cos(4x) \ = \ cos(2*2x) \ = \ 2*cos^2(2x) \ - \ 1\)
and continue.....